Path Segment in MPLS Based Segment Routing Network

Segment Routing (SR) is a source routed forwarding method that allows to directly encode forwarding instructions (called segments) in each packet, hence it enables steering traffic through a network without the per-flow states maintained on the transit nodes. Segment Routing can be instantiated on an MPLS data plane or an IPv6 data plane. The former is called SR-MPLS , the latter is called SRv6 . SR-MPLS leverages the MPLS label stack to construct an SR path. In an SR-MPLS network, when a packet is transmitted along an SR path, the labels in the MPLS label stack will be swapped or popped. So that no label or only the last label (e.g. Explicit-Null label) may be left in the MPLS label stack when the packet reaches the egress node. Thus, the egress node cannot determine along which SR path the packet came. However, to support various use-cases in SR-MPLS networks, like end-to-end 1+1 path protection (Live-Live case) , bidirectional path , or Performance Measurement (PM) , the ability to implement path identification on the egress node is a pre-requisite. Therefore, this document introduces a new segment type that is referred to as the Path Segment. A Path Segment is defined to uniquely identify an SR path in an SR-MPLS network in the context of the egress node. It is normally used by the egress nodes for path identification hence to support various use-cases including SR path PM, end-to-end 1+1 SR path protection, and bidirectional SR paths correlation.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT“, "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in when, and only when, they appear in all capitals, as shown here.

DM: Delay Measurement. LM: Loss Measurement. MPLS: Multiprotocol Label Switching. MSD: Maximum SID Depth. PM: Performance Measurement. PSID: Path Segment ID. SID: Segment ID. SL: Segment List. SR: Segment Routing. SR-MPLS: Segment Routing instantiated on MPLS data plane.

A Path Segment is a single label that is assigned from the Segment Routing Local Block (SRLB) or Segment Routing Global Block (SRGB) or dynamic MPLS label pool of the egress node of an SR path. It means that the Path Segment is unique in the context of the egress node of the SR path. When a Path Segment is used, the Path Segment MUST be inserted at the ingress node and MUST immediately follow the last label of the SR path, in other words, inserted after the routing segment (adjacency/node/prefix segment) pointing to the egress node. The Path Segment may be used to identify an SR-MPLS Policy, its Candidate-Path (CP), or a SID List (SL) terminating on an egress node depending on the use-case. The value of the TTL field in the MPLS label stack entry containing the Path Segment MUST be set to the same value as the TTL of the last label stack entry for the last segment in the SR path. If the Path Segment is the bottom label, the S bit MUST be set. Normally, the intermediate nodes will not see the Path Segment label and do not know how to process it. A Path Segment presenting to an intermediate node is an error condition. A Path Segment can be used in the case of Penultimate Hop Popping (PHP), where some labels are be popped off at the penultimate hop of an SR path, but the Path Segment MUST NOT be popped off until it reaches at the egress node. The egress node MUST pop the Path Segment. The egress node MAY use the Path Segment for further processing. For example, when performance measurement is enabled on the SR path, it can trigger packet counting or timestamping. In some deployments, service labels may be added after the Path Segment label in the MPLS label stack. In this case, the egress node MUST be capable of processing more than one label. The additional processing required can have an impact on forwarding performance. Generic Associated Label (GAL) is used for Operations, Administration and Maintenance (OAM) in MPLS networks . When GAL is used, it MUST be added at the bottom of the label stack after the Path Segment label. Entropy label and Entropy Label Indicator (ELI) as described in for SR-MPLS path, can be placed before or after the Path Segment label in the MPLS label stack. The SR path computation needs to know the Maximum SID Depth (MSD) that can be imposed at each node/link of a given SR path . This ensures that the SID stack depth of a computed path does not exceed the number of SIDs the node is capable of imposing. The MSD used for path computation MUST include the Path Segment label. The label stack with Path Segment is shown in Figure 1:

+--------------------+ | ... | +--------------------+ | Label 1 | +--------------------+ | Label 2 | +--------------------+ | ... | +--------------------+ | Label n | +--------------------+ | Path Segment | +--------------------+ | ... | +--------------------+ ~ Payload ~ +--------------------+ Figure 1: Label Stack with Path Segment Where: The Labels 1 to n are the segment label stack used to direct how to steer the packets along the SR path. The Path Segment identifies the SR path in the context of the egress node of the SR path.

Several ways can be used to allocate the Path Segment. One way is to set up a communication channel (e.g., MPLS Generic Associated Channel (G-ACh)) between the ingress node and the egress node, and the ingress node of the SR path can directly send a request to the egress node to allocate a Path Segment. Another way is to leverage a centralized controller (e.g., SDN controller) to assign the Path Segment. In this case, the controller MUST make sure (e.g., by some capability discovery mechanisms outside the scope of this document) that the egress node knows the Path Segment and it can process it, as well as the label does not collide with any label allocation done by the egress node. Path Computation Element Protocol (PCEP) based Path Segment allocation for SR Policy is defined in . Also, BGP based Path Segment allocation for SR Policy is defined in .

Binding SID (BSID) can be used for SID list compression. With BSID, an end-to-end SR path can be split into several sub-paths, each sub-path is identified by a BSID. Then an end-to-end SR path can be identified by a list of BSIDs, therefore, it can provide better scalability. BSID and Path SID (PSID) can be combined to achieve both sub-path and end-to-end path monitoring. A reference model for such a combination in (Figure 2) shows an end-to-end path (A->D) that spans three domains (Access, Aggregation and Core domain) and consists of three sub-paths, one in each sub-domain (sub-path (A->B), sub-path (B->C) and sub-path (C->D)). Each sub-path is allocated a BSID. For nesting the sub-paths, each sub-path is allocated a PSID. Then, the SID list of the end-to-end path can be expressed as <BSID1, BSID2, ..., BSIDn, e-PSID>, where the e-PSID is the PSID of the end-to-end path. The SID list of a sub-path can be expressed as <SID1, SID2, ...SIDn, s-PSID>, where the s-PSID is the PSID of the sub-path. Figure 2 shows the details of the label stacks when PSID and BSID are used to support both sub-path and end-to-end path monitoring in a multi-domain scenario.

/--------\ /--------\ /--------\ / \ / \ / \ A{ Access }B{ Aggregation }C{ Core }D \ / \ / \ / \--------/ \--------/ \--------/ Sub-path(A->B) Sub-path(B->C) Sub-path(C->D) |<--------------->|<-------------->|<-------------->| E2E Path(A->D) |<------------------------------------------------->| +------------+ ~A->B SubPath~ +------------+ +------------+ |s-PSID(A->B)| ~B->C SubPath~ +------------+ +------------+ | BSID(B->C) | |s-PSID(B->C)| +------------+ +------------+ +------------+ | BSID(C->D) | | BSID(C->D) | ~C->D SubPath~ +------------+ +------------+ +------------+ +------------+ |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| +------------+ +------------+ +------------+ +------------+ Figure 2: Nesting of Path Segments

As defined in , performance measurement can be classified into Passive, Active, and Hybrid measurement. For Passive performance measurement, path identification at the measuring points is the pre-requisite. Path Segment can be used by the measuring points (e.g., the ingress and egress nodes of the SR path or a centralized controller) to correlate the packet counts and timestamps from the ingress and egress nodes for a specific SR path, then packet loss and delay can be calculated for the end-to-end path, respectively. Path Segment can also be used for Active performance measurement for an SR path in SR-MPLS networks for collecting packet counters and timestamps from the egress node using probe messages and . Path Segment can also be used for In-situ OAM for SR-MPLS to identify the SR Path associated with the in-situ data fields in the data packets on the egress node . Path Segment can also be used for In-band PM for SR-MPLS to identify the SR Path associated with the collected performance metrics .

In some scenarios, for example, mobile backhaul transport networks, there are requirements to support bidirectional paths, and the path is normally treated as a single entity. The both directions of the path have the same fate, for example, failure in one direction will result in switching traffic at both directions. MPLS supports this by introducing the concepts of co-routed bidirectional LSP and associated bidirectional LSP . In the current SR architecture, an SR path is a unidirectional path . In order to support bidirectional SR paths, a straightforward way is to bind two unidirectional SR paths to a single bidirectional SR path. Path Segments can then be used to identify and correlate the traffic for the two unidirectional SR paths at both ends of the bidirectional path. defines procedures on how to use PCEP for SR Policy to initiate a bidirectional SR path. Also, defines procedures on how to use BGP for SR Policy to initiate a bidirectional SR path.

For end-to-end 1+1 path protection (i.e., Live-Live case), the egress node of the path needs to know the set of paths that constitute the primary and the secondaries, in order to select the primary path packets for onward transmission, and to discard the packets from the secondaries . To do this in Segment Routing, each SR path needs a path identifier that is unique at the egress node. For SR-MPLS, this can be the Path Segment label allocated by the egress node. There then needs to be a method of binding this SR path identifiers into equivalence groups such that the egress node can determine for example, the set of packets that represent a single primary path. It is obvious that this equivalence group can be instantiated in the network by an SDN controller using the Path Segments of the SR paths.

This document does not introduce additional security requirements and mechanisms other than the ones described in .

This document does not require any IANA actions.

The authors would like to thank Adrian Farrel, Stewart Bryant, Shuangping Zhan, Alexander Vainshtein, Andrew G. Malis, Ketan Talaulikar, Shraddha Hegde, and Loa Andersson for their review, suggestions and comments to this document. The authors would like to acknowledge the contribution from Alexander Vainshtein on "Nesting of Path Segments".

The following people have substantially contributed to this document: